The present invention relates to an apparatus and a method for modulating the optical intensity with a noise suppressed by using a linear optical modulator; and more Particularly, to an apparatus and a method for modulating the optical intensity a noise suppressed by using a linear optical modulator, in which, in a method for adjusting the bias voltage of an optical modulator by using the pilot signals, the noises due to the pilot signals are suppressed, so that the characteristics of signals transmitted from an optical communication system can be improved, and that the resolution of the measured physical quantities can be improved in an optical measuring instrument.
An optical intensity modulator is usefully applied not only to the optical communication field but also to the other optical technology including the optical measurements. Further, the optical intensity modulator necessarily uses the bias voltage adjusting method to improve the performance in the measurement and in the optical communication. The method in this connection is divided into: a method of measuring the optical intensity to adjust the bias voltage; and a method of using the pilot signals to adjust the bias voltage.
The method of measuring the optical intensity to adjust the bias voltage has a simple signal processing structure, and is very sensitive to external influences. Therefore, this method is widely adopted for measuring the external physical quantities.
In contrast to this, the method of utilizing the pilot signals to adjust the bias voltage is not sensitive to the external influences, and therefore, the method is widely adopted in the optical communication system which requires a stable operation. This will be described referring to FIG. 1. FIG. 1 illustrates the conventional optical intensity modulator using the pilot signals.
As shown in FIG. 1, in this conventional method for adjusting the bias voltage of the optical intensity modulator, first, the signals which are constant in the optical intensity are inputted into an optical intensity modulator 11. Then the main signals which are to be transmitted are mixed together with electrical signals which have passed through a pilot signal generator 12 for adjusting the bias voltage. Then these mixed signals are supplied to the optical intensity modulator 11.
Under this condition, the optical output signals of the optical intensity modulator consist of the main signals and the pilot signals to form the signals in which the optical intensity has been modulated. That is, they include data signals and pilot signals of, i.e., dc (direct current), fo, 2fo, 3fo . . .
That is, the optical signals which are outputted from the optical intensity modulator 11 include: dc, fo, 2fo, . . . , and data signals. However, the necessary components are the dc and data components, and therefore, the fo, 2fo, . . . components are noises.
Meanwhile, a part of the output optical signals of the optical intensity modulator 11 becomes monitoring signals to be transferred to a photo detector 14 which outputs electrical signals. Under this condition, signals which are same as the output signals of the optical intensity modulator 11 are also transferred to the photo detector 14, but they are different only in the optical intensity.
Thereafter, the magnitude of the 2fo component is measured by a negative feedback part 15 by utilizing the 2fo component of the pilot signals and the dc, fo, 2fo, . . . data components of the photo detector 14. The negative feedback circuit 15 includes a phase detector and a multiplier. If the value of the 2fo component which has been measured by the negative feedback circuit 15 is not zero, the level of the bias voltage is varied until the 2fo component becomes 0.
Now the conventional optical intensity modulator constituted as above will be described as to its bias voltage adjusting method.
The conversion of the electrical information signals to optical signals is carried out by the optical intensity modulator 11. Under this condition, if the phase difference of the optical intensity modulator 11 is not exactly 90 degrees, then the extinction ratio of the ON/OFF signals is aggravated, resulting in that noises are generated. Accordingly, in order to realize a phase difference of 90 degrees, the pilot signals of the frequency fo are supplied to the optical intensity modulator 11, thereby adjusting the bias voltage.
Thereafter, the beams which have been modulated by the pilot signals pass through an optical splitter to be detected by the photo detector 12. Thus the frequencies which correspond to integer-multiples of fo are detected. Under this condition, if the 2fo components are made to be 0 by using the negative feedback circuit 13, then the phase difference becomes 90 degrees.
However, the pilot signals other than the main signals (electrical information signals) modulate the optical signals, and therefore, the pilot signals themselves act as the noises, thereby generating noises. Accordingly, in this method using the pilot signals, the magnitude of the pilot signals has to be maintained at the minimum, but according as the magnitude of the pilot signals is reduced, the magnitude of the detected signals of the photo detector 12 becomes smaller, with the result that it becomes difficult to adjust the phase difference of the optical intensity modulator 11. Further, the 2fo component which is detected by the photo detector 12 is very much smaller than the fo component, and therefore, the magnitude of the pilot signals which are supplied to the photo detector 12 can be a serious problem.
The present invention is intended to overcome the above described disadvantages of the conventional techniques.
Therefore it is an object of the present invention to provide an apparatus and a method for modulating the optical intensity, with the noises suppressed by using a linear optical modulator, in which in a method for adjusting the bias voltage of an optical modulator by using the pilot signals, the input optical intensity is modulated by using a linear optical modulator, so that the error-inducing signal components are offset while maintaining the signal component for adjusting the bias voltage, thereby inhibiting the amplitude noises caused by the pilot signals.
In achieving the above object, the apparatus for modulating an optical intensity with amplitude noises suppressed by using a linear optical modulator according to the present invention includes: a pilot signal generating means for generating pilot signals so as to adjust a bias voltage; a phase delaying means for delaying the phase of the pilot signals after their generation by the pilot signal generating means; a linear optical modulating means for modulating optical signals from an external source by the phase-delayed signal; a signal adding means for adding together data information signals and the pilot signals from the pilot signal generating means; an optical intensity modulating means for modulating the linearly modulated signals from the linear optical modulating means to modulate the optical intensity by the added electrical signals from the signal adder and adjusting the bias voltage by negative feedback signals; a photo detecting means for detecting output signals of the optical intensity modulating means to convert them to electrical signals; and an amplitude measuring of a frequency component means for receiving the electrical signals from the optical detecting means and the pilot signals from the pilot signal generating means to measure the amplitude of the frequency component so as to generate negatively fed-back signals and so as to transfer them to the optical intensity modulating means.
In another aspect of the present invention, the method for modulating an optical intensity with amplitude noises suppressed by using a linear optical modulator according to the present invention includes the steps of: generating the pilot signal in order to adjust the bias voltage and applying the added signal to the optical intensity modulator, the added signals having been formed by combining the data signals and the pilot signals (first step); measuring the amplitude of the second harmonic frequency component from partly detected signals from among output signal and measured electrical signals to transfer negatively feed-back signals to an optical intensity modulator, the electrical signals having been converted from partly detected signals from among output signals, and then adjusting the bias voltage(second step); delaying the phase by generating pilot signals and modulating the optical signals from an external source to provide a linear component by delayed pilot signal, and suppressing the amplitude noise from the pilot signal (third step).